This invention relates generally to ultrafiltration technology and, more specifically, to a spiral wound filtration module for use in cross flow filtration and method of constructing same.
Semi-permeable membranes are utilized in both high pressure reverse osmosis and low pressure ultrafiltration and microfiltration processes. A typical design for an ultrafiltration device is to provide a spiral wrapped filtration module. Such a module is made by providing a permeate tube around which is wound one or more permeate envelopes or membrane leaves. These membrane leaves are separated by feed spacer screens. The spirally wound leaves have heretofore been held in place by a bypass spacer screen which is fused to the outside end of one layer of the feed spacer screen. The bypass screen is then wound around the circumference of the assembled leaves and ultimately fused to itself to complete the module.
The module is loaded into a housing or pressure vessel and operated at a slight pressure drop across the module. Normal flow conditions also include temperatures in the range of 80.degree.-140.degree. F. A combination of the hydraulic forces exerted on the module and the heat from the processing operation have a tendency to soften the thermoplastic materials utilized as the bypass and feed spacer screens. This may cause the spiral to loosen reducing the frictional forces holding successive material layers and ultimately causing the feed spacer screen to extrude. Such extrusion can lead to sanitation problems and low operating efficiency caused by low flow regions attributable to the extrusion problem. The increase in outer diameter which results from the extrusion also makes the modules difficult to remove for replacement, inspection or testing.
As far as is known, there has been no prior attempt to reduce or eliminate feed spacer extrusion or circumferential expansion in spiral wound ultrafiltration modules of the type described. For non-sanitary applications, the problem is avoided by an overwrap such as fiberglass-reinforced epoxy that is allowed to harden and form a rigid outer shell for the module. This is unacceptable in sanitary applications or other uses where no stagnant flow areas are permissible since stagnant flow is inherent along the surface of the hardened outer shell.
It is, therefore, an object of the present invention to provide an ultrafiltration module and method of constructing same which substantially precludes feed spacer extrusion and circumferential expansion by circumscribing the module with one or more restraining bands.
As a corollary to the above object, it is an objective of this invention to substantially reduce or eliminate feed spacer extrusion and circumferential expansion without creating dead space which would adversely impact hygienic conditions or reduce operating efficiency.
Another object of the invention is to provide an ultrafiltration module which is not subject to feed spacer extrusion or circumferential expansion and a method of constructing same which will not require modification of existing pressure vessels that comprise a part of typical filtration systems.
It is also one of the objectives of our invention to provide an ultrafiltration module and method of constructing it which will not undergo significant feed spacer extrusion or circumferential expansion and which can be utilized with modules having one or a plurality of membrane leaves.